Adherent metal coatings on rubber-modified epoxy resin surfaces

ABSTRACT

A method of forming adherent metallized coatings on a substrate which is useful for the manufacture of printed circuit boards as well as other metal coated articles involves providing the substrate with a rubber-modified epoxy surface or coating, sputter etching at least 50 A. from the surface followed by vacuum depositing an adherent thin metal film of Cr, Ni, Ni-V alloy, Pt, Pd or Ti onto the substrate. Another metal layer is then provided over the adherent thin film.

TECHNICAL FIELD

This invention relates to the manufacture of adherent metal coatings onpolymer surfaces and more particularly to the formation of such coatingson epoxy surfaces.

BACKGROUND OF THE INVENTION

In many applications, it is desirable to provide strong adherent metallayers on a polymer substrate. Examples of such uses are widelydiversified. For example, adherent metal coatings on polymer substratescan be used in printed circuit boards in the electronics,telecommunications, computer and other industries for creatingconductive paths on interconnector devices, or it may be used merely asa radio interference barrier. Such coatings also have applications inthe manufacture of certain types of credit cards, video disks and fordecorative coatings.

Various methods have been employed for providing metal coatings onpolymer surfaces. Such methods include lamination by means of anadhesive, electroless plating, and vacuum metallization. The lattermethod includes vacuum evaporation, sputter deposition and ion plating.While vacuum metallization has become a desirable and cost efficientmethod of applying metallic coatings onto polymer substrates, theadhesion of such films to the substrates is often less than adequate,especially when metallizing durable high performance polymers such asepoxy resins. The adhesion problem often becomes particularly criticalwhere the vacuum metallized layer is further processed by electrolessmetal plating or electroplating to increase the thickness of themetallized layer. In such cases, the adhesion of the vacuum metallizedfilm must withstand the environment and temperatures of subsequentlyemployed plating baths.

SUMMARY OF THE INVENTION

A substrate having a rubber-modified or highly unsaturated epoxypolymeric surface is first treated to remove any weak boundary layers onthe surface. A thin adherent base film of a metal selected fromtitanium, chromium, nickel, nickel-vanadium alloy, palladium or platinumis then vacuum deposited on the treated surface. The thin layer acts asa glue or adhesion promoting layer for a substantially thicker film of aprimary metal which is deposited over the thin base film.

It should be understood that the metallized surface as described abovemay be provided as a blanket metallization over the entire surface ofthe substrate or may be provided over only portions of the substrate soas to depict a predetermined metallized pattern on the substrate.Alternatively, a pattern may be formed by various processing steps wellknown in the art subsequent to either a blanket vacuum deposition of thethin base film or subsequent to blanket electroless plating,electroplating or vacuum deposition of the primary metal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1c are partial, elevational, cross-sectional views of a printedcircuit board at various stages during one manufacturing process; and

FIGS. 2a-2c are partial, elevational, cross-sectional views of a printedcircuit board at various stages during a second manufacturing process.

DETAILED DESCRIPTION

For convenience, the invention will be described in terms of preparingbare printed circuit boards and finished printed circuit boards byadditive and semiadditive processing. However, it should be understoodthat the invention extends to many other uses other than printed circuitboards and is suitable for any use wherein an adherent metal coating isapplied over a suitable epoxy polymeric substrate.

In general, the invention comprises treating the surface of thesubstrate to be metallized so as to remove any weak boundary layers fromthe surface which inhibits adhesion and then vacuum depositing a thinadherent base film of a metal selected from Ti, Ni, Ni-V alloy, Cr, Pd,or Pt and then applying a thicker coating such as by means of vacuummetallization, electroless or electroplating techniques, or acombination thereof to form a thicker metal layer over the thin basefilm.

It has unexpectedly been found that the novel process is unique forrubber-modified epoxy or highly unsaturated epoxy surfaces in thathighly adherent metal films (films having 90° peel strengths of at least8 lbs. per inch) cannot be formed over other polymers by this method.Attempts to metallize other polymer substrates, e.g., epoxies withoutrubber or without a high degree of unsaturation, polycarbonates,acrylonitrile-butadiene-styrene, lacquer and polyacrylates wereunsuccessful. Unexpectedly, the only substrates which appear to beparticularly suited to the formation of an adherent metal coating (≧8lbs./in. peel strength) by means of this invention are theaforementioned type of epoxy resin surfaces. In the past, such epoxypolymers were not amenable to vacuum metallization of adherent films. Anexample of suitable rubber-modified epoxy resins are those which containbutadiene acrylonitrile rubber in the formulation. The rubber isgenerally present as a dispersed separate phase in the epoxy. An exampleof a highly unsaturated epoxy is one that has at least 51/2 weightpercent of carbon-to-carbon double bonds based upon the total weight ofthe polymer excluding phenyl ring unsaturated carbon atoms, e.g., Epon872, a commercially available epoxy resin flexibilized by pre-reactionwith a dimer acid which has unsaturation and is incorporated in thepolymer chain. The dimer acid has the formula ##STR1## In order toremove the weak boundary layer which is generally present on suchsubstrates, one may employ sputter etching techniques or mechanicalabrasion. Sputter etching is preferred and at least about 50 A. of thesurface should be removed. It may be noted that alternatively as much as10⁷ A. may be removed such as by mechanical abrasion if desired. Thethin base film is generally deposited to a thickness in the order of1,000 A. and typically from 100 A. to 3,000 A. although thicker filmsmay be employed. Any vacuum metallization technique is suitable. Forexample, one may use vacuum evaporation, sputter deposition or ionplating to form the thin base film; sputter deposition or ion platingbeing preferred.

In the manufacture of printed circuits as well as in the manufacture ofother products, it has become desirable to eliminate complex steps,where possible, for the reasons of cost savings and also to eliminate oralter steps which may potentially cause environmental problems. In thecase of the manufacture of printed circuit boards by either subtractiveor additive processing as is described in U.S. Pat. Nos. 3,673,680,3,296,099, or 3,679,472 which are incorporated herein by reference,chemical solutions are required to either treat the substrate to make itconductive for electroless deposition, or acidic and corrosive etchingsolutions are required to etch thick copper layers (such as in thesubtractive techniques) in order to form the printed circuits. Further,other wet chemical techniques are needed to sensitize surfaces in theadditive processing techniques. In accordance with the instantinvention, a simplified method is presented which is environmentallycleaner and provides a metal coating on an epoxy polymer surface havingan adhesion at least equal to that of prior art techniques, (i.e., ≧5lbs./in peel strength. By employing the novel method, printed circuitscan be formed by additive or semiadditive techniques which requirelittle or no etching and which reduce the use of solvents and variousoxidizing agents. Examples of some printed circuit board structures areshown in the accompanying FIGS.

In accordance with the novel method for making a printed circuit board,one starts with a substrate member 10 such as an epoxy orfiberglass-epoxy member provided with spaced through-holes 12 thereinand which preferably has a rubber-modified epoxy coating 14 on itssurface typically about 0.002 inches thick. The rubber-modified epoxymay, for example, be selected from any of the compositions disclosed byR. B. Lewis and T. A. Giversen in U.S. Pat. No. 4,176,142 which isincorporated herein by reference. We have found that such surfaces havea weak boundary layer which prevents strong adhesion by vacuummetallized coatings. This boundary layer at the surface of the substrateis removed by preferably placing the substrate in a sputter etch vacuumapparatus so as to remove at least 50 A. of the outermost surface of thepolymer. Either RF or DC sputter etching may be employed. Typically, forRF sputter etching, a wattage in the order of 500 watts at 13 to 14 MHzwith an argon pressure of from 5 to 100 microns (approximately 10microns being preferred) is suitable. Usual etch time in order to etchat least 50 A. from the surface is approximately 3 minutes under theseconditions. It should be understood that the conditions for sputteretching are given merely by way of example and the actual conditions forsputter etching are not critical. Subsequent to sputter etching, a thinadherent film 16, preferably in the order of 100 A., e.g., 30-200 A. ofa metal selected from the group consisting of Cr, Ni, Ni-V alloy, Pd, orPt is vacuum deposited such as by means of well known sputtering or ionplating techniques such as are described in U.S. Pat. No. 4,166,018issued to J. S. Chapin or in "Vacuum Metalizing," Metal Finishing by P.R. Forant, November, 1979, pages 17-20, both of which are incorporatedherein by reference. This is preferably done without breaking the vacuumafter completing sputter etching. After vacuum deposition, whichprovides a metal-polymer interface 18 having superior adhesion,additional metal 20, typically copper for printed circuit boards, can bedeposited by means such as by electroplating, electroless plating orvacuum evaporation onto the previously vacuum deposited thin metallayer. This second layer is generally substantially thicker than thethin adhesion promoting metal film 16. When electroless plating isemployed the thin adherent base metal film 16 is preferably Cr and whenelectroplating is to be employed to build up the primary layer, a thinadherent base layer 16 of Ni or Ni-V alloy is preferred.

It may be noted that the prior art, such as U.S. Pat. Nos. 4,121,007 and4,193,849, has claimed that good adhesion can be obtained by sputtering,ion plating or otherwise vacuum depositing metals such as copperdirectly onto polymers for constructing useful articles such as printedcircuits without the need for applicants' initial steps. However, wehave found that this is not true when a high performance polymer, e.g.,epoxy, is to be coated. We have unexpectedly discovered that byemploying a rubber-modified or highly unsaturated epoxy surface,removing at least 50 A. of the outermost portion of this surface, andproviding a thin intermediate adhesive or glue layer selected from aspecific small group of metals, consistently good adherent films can beformed. While the elimination of these steps is sometimes possible, theresults without the novel steps are variable from one sample to anotherand therefore unreliable. Furthermore, the bonds achieved without thenovel steps as taught herein are excessively subject to degradation fromthe long term effects of atmospheric humidity and of agents which areoften necessary during later processing steps in the manufacture ofcompleted articles such as printed circuits. Examples of agents whichadversely affect metals deposited by prior art vacuum depositiontechniques, while they do not substantially affect vacuum depositedmetals formed in accordance with this invention, are such things as theheat which is used to harden screen printing ink during patterning andthe methylene chloride or other solvent used for stripping ofphotoresist as well as electroless and electroplating baths.

It has unexpectedly been found that the novel treatment for insuringgood adhesion of the metal coating on the polymer substrate is not onlylimited to the use of an epoxy substrate having a surface which containsa rubber or an epoxy having a high degree (at least 5.5 weight percent)of unsaturated carbon atoms excluding aromatic double bonds aspreviously indicated, but it has also been found to be of extremeimportance to remove at least 50 A. of the surface of the substrate inorder to attain the desired adhesion. Unless this minimal amount ofsubstrate is removed by either sputter etching or mechanical means aspreviously set forth, desired adhesion is not reliably attainable. Itmay be noted that mechanical abrasion requires that substantially morethan 50 A. be removed to insure elimination of the weak boundary layer.

It has further been discovered that the requisite adhesion is onlyattained with the thin metal bonding or glue layers as set forth herein.The use of other metals as a substitute for the specified glue layermetals results in poor adhesion. For example, if one attempts to usemetals such as copper, aluminum or gold as the thin adhesion promotinglayer, adhesion is poor.

A typical suitable epoxy surface for processing in accordance with thisinvention is an epoxy resin which contains from 5 to 15 weight percentof an acrylonitrile butadiene rubber such as Goodrich X-8 or X-13rubbers. It may further be noted that while the thin metal glue layer 16is generally applied in thicknesses of the order of 100 A., thicknessesof from 30 A. to 10,000 A. are also suitable and have resulted in metalcoatings having more than adequate adhesion to the substrate. It hasfurther been found that processing parameters such as sputtering voltageand argon pressure are not critical. It may be noted that whileroughening and/or deglossing is known as a method for promoting adhesionof a coating on a substrate, the removal of 50 A. or more of the surfaceby the preferred method of sputter etching does not necessarily roughenor degloss the surface. Analysis of the surface by means of x-rayphotoelectron spectroscopy indicates that the sputter etching stepremoves a hydrocarbon from the surface which exposes the underlyingrubber containing material. It may also be noted that while the ABSpolymer, which is a frequently used plastic, contains rubber, the novelprocess does not result in a strongly adherent metal coating on such apolymer substrate. Consequently, it presently appears that the novelprocess is unique for rubber-modified epoxy and epoxies having a highdegree of unsaturation in the polymer chain.

The novel process can be employed in the manufacture of printed circuitboards by means of various types of processing including ultra thin cladprocessing, semiadditive processing and additive processing.

In accordance with the manufacture of printed circuit boards by means ofultra thin clad processing, typically a fiberglass-epoxy substrate 10without copper cladding is powder coated with a rubber-modified epoxyresin 14 on each side to a thickness of approximately 0.002 inch thickto provide an adhesion-promoting surface. The coated panels are thendrilled to provide through-holes 12 and scrub-deburred. In a vacuummetallization chamber, three things are then done: (a) sputter-etchcleaning so as to remove at least 50 A. of the outer surface, (b)deposition of approximately one microinch layer of an adhesion promotingmetal 16 selected from Ni, Ni-V alloy, Pd, Pt, Cr and T, and (c)deposition of about a 40 microinch thick copper layer 20. The adhesionpromoting layer and copper layer coat the insides of the holes and thetwo major surfaces of the substrate.

At this point, the circuit board is similar to a purchased subtractivethin clad panel, except that the metallization is only about 40microinch thick instead of the usual 525 microinches of copper.

The next steps are essentially the same as those used for standardsubtractive material, which include the application, exposure anddevelopment of photoresist 22 for obtaining the desired pattern,electroplating a thick copper layer 24 to build up the desired circuitpattern, preferably plating a tin alloy etch resist 26 over the copper24, stripping the photoresist 22 (FIG. 1b) and etching of the thin cladcopper 20 and underlying adhesion promoting metal layer 16 (FIG. 1c). Bythis method, etching times can be reduced in order to minimize theundercutting of fine lines due to the extremely thin initial copper cladlayer. In addition, the amount of etchant required is minimized.

In accordance with the partially additive processing, the same procedureis followed as described in the ultra thin clad processing method exceptthat the thick copper layer 24 is built up by electroless platinginstead of electroplating and no tin alloy plating 26 is used. Thisprocess offers the advantages of high yield of fine line due to the factthat electroless full plate offers thickness uniformity from panelcenter to panel edge which is not generally attainable byelectroplating. Also, the resist tends to prevent uncontrolled lateralgrowth of conductors and the ultra thin initial metallization etchesquickly, minimizing undercutting and the amount of etchant required.

Referring to FIGS. 2a-2c, in accordance with fully additive processing,the substrate 110 would be powder coated 114, drilled to formthrough-holes 112 and scrub-deburred as described above with referenceto ultra thin clad processing. A photoresist pattern 122 in the negativeof the desired circuit pattern is then applied to the coated substrate.The substrate is then placed in a vacuum chamber where it is sputteretch cleaned so as to remove at least 50 A. from the surface of thesubstrate coating 114 and an adhesion promoting metal layer 116 is thendeposited over the entire substrate in a thickness of approximately 1microinch followed by the vacuum deposition of a 10 microinch layer ofcopper 120. It may be noted that the copper layer 120 may be applied byflash electroless plating techniques instead of vacuum deposition ifdesired. Also, other metals such as Ni, can be used in place of copper.Only 10 microinches of copper are necessary since electricalconductivity is not of import due to the fact that electroplating is notrequired. The photoresist 122 is then stripped, which removes the thinmetal layers 116 and 120 from regions where the resist had been.However, the vacuum metallized adhesion promoting layer 116 and copperlayer 120 thereover are not removed from regions where there was noresist. The thickness of the remaining metal is then built up to thefull desired thickness of approximately 0.0014 inches by electrolesscopper 124 deposition. Any thickness which gives suitable electricalconductivity for the circuit is suitable.

Printed circuit boards made in accordance with this invention generallyexhibit 90° peel strengths of at least 8 pounds per inch. It should benoted that such high peel strengths are not obtained with othersubstrates or where the process is altered such as where the weaksurface layer is not removed prior to vacuum metallization of the thinadhesion promoting metal layer or where another metal is substituted forthe metals taught as useful for the thin metal adhesion promoting layer.Further, as long as the above factors are conformed to, large variationsin process operating parameters are acceptable. For example, curingtimes and temperatures for the rubber-modified epoxy coating materialare not critical. It has also been determined that large variations incuring agent amount and type, the amount and type of rubber materialadded to the epoxy, the thickness of the adhesion promoting metalliclayer, the sputter etch and sputter deposition or other vacuumdeposition parameters and the like can be varied over substantial rangesand are not critical. More specifically, peel strengths of greater than8 pounds per inch and typically greater than 10 pounds per inch wereobtainable when using a rubber-modified epoxy material intentionallyovercured at 200° C. for 45 minutes as compared to the usual curingtemperature and time of 175° at 20 minutes, or when the amount of curingagent was varied from 1% through 8.5%, or when the amount of rubber inthe epoxy formulation was varied between 4.5 and 16%. It was also foundthat fairly long delays of 6 hours or more between weak boundary layerremoval by sputter etching and the step of sputter metallization did notadversely effect the peel strengths. This indicates that the weakboundary layer material being removed is substrate material itselfrather than merely adsorbed surface gas.

It should be noted that magnetron sputtering, is the preferredsputtering method since it is much more efficient than non-magneticallyenhanced modes. However, magnetron sputtering cannot be used to depositmetals such as nickel which are magnetic. Such magnetic metals wouldattract magnetism to the substrate distorting the optimum fieldconfiguration. We have discovered that nickel, which is a generallypreferred metal adhesion promoting material, can still be employed withmagnetron sputtering when used in the form of a non-magnetic alloy with8 weight percent vanadium.

When employing the fully additive process as previously described, thethin metallic glue layer metallizations that were successful for theultra thin clad processing and the partially additive processing did notgive consistently good adhesion through the electroless buildup to0.0014 inches of copper. In many cases the metals blistered duringelectroless deposition and peel strengths were less than 5 pounds perinch both before and after baking subsequent to deposition. Attempts tocure this problem by altering the electroless plating bath wereunsuccessful. It was unexpectedly discovered together with a co-worker,however, that the attack causing reduction of adhesion by theelectroless plating bath can be substantially completely alleviated bybaking the substrate in air, for example, at about 120° C. for about 30minutes prior to sputter etching. Preferably the material is not allowedto cool completely to room temperature between baking and sputteretching. It is hypothesized that this baking removes absorbed gases fromthe surface of the substrate. By placing the still warm substrate in thesputtering apparatus and pumping down prior to complete cooling, gasescan be prevented from readsorbing on the surface. For example,substrates having a rubber-modified epoxy surface were first baked, andthen placed warm in a sputtering apparatus and sputter etched, and thensputter metallized with chromium and removed from the vacuum apparatus.Thereafter, a 0.0014 inch copper layer was built-up by placing in aMcDermid 9620 electroless plating bath. Peel strengths ranging between 8to 10 pounds per inch were obtained. It is also preferred, but notnecessary, to bake again subsequent to the completion of all vacuummetallization steps. Experiments done to seek alternatives to the bakingas a solution for the blistering problem when the substrate is to beelectrolessly plated failed to result in good adhesion. For example,placing the substrate in a desiccator for one week at room temperaturefollowed by sputter etching and sputter metallization did not provideadequate adhesion. Similarly, placing the sample for an extended period,e.g., three hours in vacuum at 10⁻⁶ Torr at room temperature, alsoresulted in poor adhesion. Consequently, whenever electroless plating isused to further buildup the thickness of the metal pattern, a pre-bakeis essential.

It has also been found that it is preferred in order to achieve moreuniform plating of through-holes, that during vacuum metallization thesubstrate should be spaced from its carrier plate, spacings of from1/4-3/8 of an inch were preferred.

It is to be understood that the above-described embodiments are simplyillustrative of the principles of the invention. Various othermodifications and changes may be devised by those skilled in the artwhich will embody the principles of the invention and fall within thespirit and scope thereof.

What is claimed is:
 1. An article of manufacture comprising:(a) asubstrate having a surface of a rubber-modified epoxy resin, saidsurface having any weak boundary layer thereon essentially removed; (b)a thin adherent base metal film selected from the group consisting ofCr, Ni, Ni-V alloy, Pd, Pt, or Ti on said surface, said metal havingbeen vacuum deposited on the surface; and (c) a thicker primary metaldeposited over the base metal film said primary metal having a peelstrength of at least 8 pounds/inch.
 2. The article recited in claim 1wherein said rubber-modified epoxy resin contains anacrylonitrile-butadiene type rubber.
 3. The article recited in claim 1wherein the weak boundary layer was removed by means of sputter etchingat least 50 A. from the surface of the substrate.
 4. The article recitedin claim 3 wherein the thickness of the base metal is from 30 A. to10,000 A.
 5. The article recited in claim 3 wherein the thickness of thebase metal film is in the order of 1,000 A.
 6. The article recited inclaim 1 wherein the primary metal is selected from the group consistingof copper, nickel or a mixture thereof.